1. Field of the Invention
The invention relates to apparatus for measuring formation pressure in a most informative and efficient manner and which is adaptable to be tubing conveyed. More particularly, the invention relates to formation pressure measurement apparatus which is tubing conveyed, therefore presenting substantially no pressure drop to the fluids flowing therethrough, and which provides a maximum of useful data with relatively little wasted data. Even more particularly, the present invention relates generally to apparatus for initiating the operation of pressure gauges which are used in measuring oil well formation or bottom hole pressures.
2. Description of the Background
Pressure is probably the most important directly measurable parameter of an oil reservoir. From pressure measurements many other important variables can be derived which can be used in evaluating a well or reservoir, such as method of production, effectiveness of secondary recovery processes, etc. Reservoir Engineers can determine effective permeability, porosity, effective drainage radius, extent of well bore damage (if such exists), continuity of aquifer, oil zone or gas cap from various pressure measurements. These provide transient pressure measurements under draw down, or build-up conditions and interference effects in a shut-in well due to a change in production or injection in another well.
In drill stem testing, special formation test equipment is mounted on the end of the drill string and lowered into the hole to a point above or adjacent to the horizon to be tested.
Drill stem tests are made to ascertain the potential productivity of a penetrated zone, to assess formation damage, to determine native reservoir pressures, and to obtain fluid samples (surface and/or subsurface). Such tests can also be used to confirm the effectiveness of water shutoffs and to determine the capability of perforations to admit fluids freely to the hole.
A drill stem test involves the measurement of bottom hole pressures with the formation to be tested alternatively closed-in and open to flow to the surface. Therefore, the equipment in the overall assembly consists of the pressure-recording device, the flow-control valves, one or more gland-type packers, and various other safety and control mechanisms. Surface equipment may also include pressure and flow measuring and control devices, along with necessary manifolds, tanks, separators, burners, etc. The test equipment or tool is made up on the drill string and set down on the bottom of the hole. A packer, an expandable hard-rubber sealing element, seals off the hole below it by expanding it with various setting techniques. A valve is opened, and any formation pressure and fluids present enter the tool. A recorder in the tool makes a recording of the formation pressure. Then the packer is released and the tool retrieved back to the surface. By looking at the record of the down hole pressure, a good indication of the characteristics of the reservoir can be obtained.
Applicant's present recording system provides delta time, probe temperature and pressure with accuracy and sensitivity for draw-down tests, build-up tests, interference tests, static tests, gradient tests, variable flow rate tests, and drill stem tests for example. These pressure records can be used to make a thorough analysis of any type of reservoir.
The recording system is battery powered, completely self-contained and records delta time, probe temperature and pressure for each record and stores it in memory. The recorder samples and records temperature and downhole pressure data for up to 60 days, depending on the batteries and sample rate selected.
The recording system can be programmed for a non-recording "delay time" of up to 17 hours to allow time for the probe to reach a desired depth before starting to collect data samples. After the delay time, the gauge can be programmed to shift to a "fast sample" mode (samples every 15 sec.) for up to 7 hours, after which it goes to the pre-selected sample interval (0.5 minutes to 64 minutes). Programming for the delay time, fast sample time and standard sample interval is done at the wellsite prior to putting the equipment downhole. This procedure is critical since the recorder capacity is only 2,000 samples.
Intelligent gauges (gauges which can be energized after a selected time interval) are extremely critical on production tests and on drill stem tests but many downhole problems occur while conducting drill stem tests. Intelligent gauges cannot be controlled after insertion into the borehole. The opportunity to activate a tool from surface, therefore, may have considerable advantages over even an intelligent gauge. For example, many cases have occurred where the drill string has been tripped in the well to one-half or three-quarters of intended depth when ice, bad weather, or rig servicing has required that operations be shut down for an extended period of time. If an intelligent gauge was programmed for a fixed delayed start with fast sample rate, and an extended delay occurred, very little recorder operating time (data samples) would be available for the critical part of this test. To compensate for this, four to eight minute sample modes are utilized to ensure complete test results, i.e. data samples during the entire test. However, this resolution will not adequately satisfy customer needs and a downhole tool with much greater memory (or the capability to activate at a desired time, i.e. upon reaching the test zone) to ensure high data rate over a given time period is needed. The present actuator switch is a direct result of this need and offers a pressure actuator switch capable of being triggered by applying a specific pressure to the tool after the target zone is reached. The pressure actuator switch of the present invention will effectively put the control of a drill stem test downhole data acquisition back on surface.